Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing my new design.

The dot dashed broken line showing the display screen or portion thereof forms no part of the claimed design. The dashed broken line showing portions of the graphical user interface forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or a portion thereof with a graphical user interface, as shown and described. 